Cassio is a Florentine gentleman soldier, a man of high manners and theoretical learning, and one of Othello's chief lieutenants. There is a supposed rivalry between Cassio and the play's villain, Iago. Iago claims to resent Cassio because Othello chose Cassio rather than Iago as his lieutenant, in spite of the fact that Cassio has no practical knowledge of battle. Iago uses Cassio in his scheme to destroy Othello; Iago insinuates throughout that Cassio is having an affair with Othello's wife, Desdemona. Othello's jealousy is eventually stoked by Iago into homicidal rage.
"Reputation, reputation, reputation! O, I have lost my reputation! ... My reputation, Iago, my reputation!"
— Cassio, [2.3.156-159][1]
In the second act, Cassio's life is nearly ruined by Iago's cunning and his own foolishness. Iago tricks Cassio into getting drunk and then incites his friend Roderigo to start a brawl with Cassio. The Cypriot governor Montano tries to end the fight by stepping between the two men, and Cassio, now blind drunk, strikes out at him. As a result, Cassio loses his lieutenancy.
Later in the play, Iago persuades Roderigo to assassinate Cassio, and together they arrange an ambush. Roderigo attacks Cassio by surprise. Cassio retaliates and mortally wounds Roderigo, but is himself stabbed from behind by Iago. His leg is wounded, but he survives. Iago then kills the wounded Roderigo. Before committing suicide, Othello apologizes to Cassio for believing Iago's lies about him.